Big fish in a…

Is it true that goldfish (or other captive fish) grow in proportion to the size of the tank they live in? I have a small goldfish in a tank at home but have seen much larger goldfish in ponds and believed they were a different species. But a friend insists they are the same, with the size difference being caused only by the volume of the body of water in which they live. Is my friend right? And if so why?

• It depends. Some Koi, a related but different species, are easily confused with large goldfish. Real goldfish have been bred and cross-bred for more than 1000 years, and are now regarded as a single species. But there are various breeds that differ in size, even allowing for pond volume.

Water supply is seldom a constraint on marine fish, but ponds and streams are another matter. Freshwater fish are ...

To continue reading this article, subscribe to receive access to all of newscientist.com, including 20 years of archive content.

To continue reading this article, log in or subscribe to New Scientist

App + web

Web

Smartphone

Tablet

$25.99 - Save 65%

12 issues for $2.17 per issue

with continuous service

Print + web

Print

Web

$28.99 - Save 61%

12 issues for $2.42 per issue

with continuous service

Print + app + web

Print

Web

Smartphone

Tablet

$39.99 - Save 73%

12 issues for $3.33 per issue

with continuous service

Web

Web only

$49.99

30 day web pass

Prices may vary according to delivery country and associated local taxes.